Proper Maintenance and Storage of pH Meter Probes

‘There are few diagnostic tools more useful to an indoor gardener than an accurate pH meter.’  -Ben Holmes, Founder Centennial Seeds

For indoor cannabis gardeners the ability to monitor pH at the root zone or in hydroculture solutions is essential for the production of healthy crops. Most plants grow best within a specific range of pH conditions. For cannabis in hydroculture that range is from 5.8 – 6.2. There will be some plants that do better toward the low end of this range and others that will thrive at the high end.

The purchase of a pH meter is a highly personal and potentially costly matter. At the low-end there are small, highly portable pH ‘pens’. These tend to be fragile and when abused or poorly cared for the pens will fail and have to be discarded.

A more durable type, one designed for longevity and serviceability is a meter with a remote probe connected by a cable to the device. I prefer this type and have used one made by Milwaukee Instruments for over three years. I have not had to replace the probe or had the unit serviced. To be fair, I have replaced the 9-volt battery once.

Regardless of which meter you choose, the secret to extending the lifetime of your instrument and maintaining its accuracy is proper cleaning, storage and regular calibration. This may sound complicated, but it’s not really. In order to understand how to take care of your new meter, let’s take a look at a typical probe.

This one is a little dirty, but it still functions very well. What you are looking at and what is common to most pH meter probes is a thin walled glass bulb filled with a concentrated electrolyte solution. Inside the bulb is a conductive wire electrode.

Surrounding the bulb in this probe is a thin red porous border. This is called the union and it is conductive as well. A wire from the union back to the processing unit completes a circuit passing through the solution being measured. The meter compares the conductivity between the two phases and calculates a value along the pH scale.

To maintain the probe it must be cleaned at least once a month, after each use and before it is stored. Most manufacturers of meters make a cleaning solution. These are typically a hydrochloric acid solution with a pH of ~ 1.

Storing a probe correctly means first cleaning the probe and then submerging the bulb in a clean vessel of storage solution. Storage solutions are a high-molarity potassium chloride solution that matches or exceeds the concentration of the solution in the bulb.

Storing the bulb in this solution prevents the solution in the bulb from being diluted over time by osmosis. Cleaning the probe before storing it prevents the buildup of algae on the union and on the bulb’s surface. Algae buildup is a major cause for failure in electrode probes.

In order to make accurate measurements of pH a meter must be calibrated to two or three known points along the pH scale. For our purposes we calibrate to the known points of 4.01 and 7.01, which will allow our meter to accurately measure any point in between these values and beyond.

Individually packaged sachets of calibration standards make the process of calibrating your instrument simple and easy.

A pH meter probe that is properly maintained and stored should provide years of trouble-free service. Knowing your instrument is in top working order gives you the confidence to trust the reliability of its readings when making critical decisions in the garden.

I hope you find this information useful. I welcome your comments.

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